1. Field of the Invention
The present invention relates to a surface emitting laser device and an atomic oscillator.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser that is capable of emitting light in the direction orthogonal to its substrate. When compared with an edge emitting semiconductor laser that emits light in the direction parallel to its substrate, the VCSEL may have some advantageous features such as lower cost, lower energy consumption, smaller size, higher performance, and easier application to two-dimensionally integrated devices, for example.
A typical surface emitting laser has a resonator structure that includes an active layer including a resonator region, an upper reflector that is arranged on an upper side of the resonator region, and a lower reflector that is arranged on a lower side of the resonator region (See e.g., Japanese Laid-Open Patent Publication No. 2008-53353). In order to obtain light with an oscillation wavelength λ, the resonator region is arranged to have a predetermined optical thickness so that light with the wavelength λ may oscillate at the resonator region. The upper reflector and the lower reflector are multi-layer structures that are formed by alternately layering materials with varying refractive indexes. That is, the upper reflector and the lower reflector comprise Distributed Bragg Reflectors (DBR) that are formed by alternately laminating a low refractive index layer and a high refractive index layer. The optical film thicknesses of the low refractive index layer and the high refractive index layer are arranged to be λ/4 so that high reflectance may be obtained at the wavelength λ.
A multi-wavelength surface emitting laser device that has a plurality of surface emitting lasers for emitting light at different wavelengths formed within a chip is known, and such a multi-wavelength surface emitting laser device is expected to be used in various fields such as wavelength-division multiplexing (WDM) (See, e.g., Japanese Patent No. 2751814, Japanese Laid-Open Patent Publication No. 2000-58958, Japanese Laid-Open Patent Publication No. 11-330631, and Japanese Laid-Open Patent Publication No. 2008-283129). As for the method of manufacturing such a multi-wavelength surface emitting laser device, for example, Japanese Patent No. 2751814 discloses alternately laminating two different types of materials to form a wavelength adjustment layer, using two different types of etching solutions that are capable of etching the materials to partially remove the materials of the wavelength adjustment layer, and arranging the wavelength adjustment layer to have different film thicknesses. Because such a wavelength adjustment layer is formed within a resonator region, by arranging the wavelength adjustment layer to have different film thicknesses, the optical thickness of the resonator region may be different for each surface emitting laser. Such a method of manufacturing the surface emitting laser device may be advantageous in view of wavelength controllability and ease of manufacture, for example.
According to the method disclosed in Japanese Patent No. 2751814, after partially removing the wavelength adjustment layer made of semiconductor materials one layer at a time through wet etching to form the surface emitting lasers, an upper reflector and a contact layer are formed on the wavelength adjustment layer through crystal growth of the semiconductor material. The contact layer is connected to one electrode and the other electrode is formed on the rear face of the substrate. However, in a surface emitting laser device having such a structure, because the wavelength adjustment layer is formed by alternately laminating different materials, when a current flows through the wavelength adjustment layer, electrical resistance may increase due to band discontinuity at the interfaces of the different material layers. Also, because the wavelength adjustment layer is arranged to have different thicknesses for the surface emitting lasers, there may be variations in the electrical resistance of the surface emitting lasers as well.
Accordingly, a surface emitting laser device is preferably structured so that each surface emitting laser has a wavelength adjustment layer and an active layer into which a current may be injected so that the current does not flow through the wavelength adjustment layer. Also, when a contact layer is formed at a position close to the wavelength adjustment layer, the contact layer may not necessarily be positioned at a node of an optical standing wave within the laminated structure, and in such case, adequate laser performance may not be obtained.